linux-kernel - Re: [PATCH v2] ftrace: Document filter and option limitations

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Date:	Wed, 5 Nov 2014 10:23:12 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ftrace: Document filter and option limitations

On Tue,  4 Nov 2014 12:46:05 -0500
Peter Hurley <peter@...leysoftware.com> wrote:

> When using the function or function_graph tracers from the command
> line, certain command line options have limitations.
> 
> Document that only kernel built-in functions can be filtered via
> ftrace_filter= or ftrace_graph_filter=. Document that tracer-specific
> options cannot be set on the command line via trace_options.
> Document that ftrace cannot do late binding for function
> filters.
> 
> Signed-off-by: Peter Hurley <peter@...leysoftware.com>
> ---
>  Documentation/kernel-parameters.txt | 6 ++++++
>  Documentation/trace/ftrace.txt      | 7 +++++--
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 4c81a86..d098ead 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -1121,6 +1121,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  			list of functions. This list can be changed at run
>  			time by the set_ftrace_filter file in the debugfs
>  			tracing directory.
> +			Only kernel built-in functions can be filtered.
>  
>  	ftrace_notrace=[function-list]
>  			[FTRACE] Do not trace the functions specified in
> @@ -1134,6 +1135,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  			function-list is a comma separated list of functions
>  			that can be changed at run time by the
>  			set_graph_function file in the debugfs tracing directory.
> +			Only kernel built-in functions can be filtered.
>  
>  	ftrace_graph_notrace=[function-list]
>  			[FTRACE] Do not trace from the functions specified in
> @@ -3521,6 +3523,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  
>  			      trace_options=stacktrace
>  
> +			Tracer-specific options are ignored when set this way.
> +			For example, the 'func_stack_trace' option cannot be
> +			set here.
> +
>  			See also Documentation/trace/ftrace.txt "trace options"
>  			section.
>  
> diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
> index 4da4261..d540273 100644
> --- a/Documentation/trace/ftrace.txt
> +++ b/Documentation/trace/ftrace.txt
> @@ -188,7 +188,8 @@ of ftrace. Here is a list of some of the key files:
>  	in with practically no overhead in performance.  This also
>  	has a side effect of enabling or disabling specific functions
>  	to be traced. Echoing names of functions into this file
> -	will limit the trace to only those functions.
> +	will limit the trace to only those functions. Only already-
> +	loaded functions can be filtered.

I would add a little more. Something that states that modules with the
same function names that are loaded at a later time will not be
filtered with theses names.

Hmm, actually, I think this could be rather trivial to implement
something that will filter modules that are added.

I think that may be a better solution. I'm assuming you would like to
have functions filtered via the command line to filter on modules as
well. Correct?


-- Steve


>  
>  	This interface also allows for commands to be used. See the
>  	"Filter commands" section for more details.
> @@ -208,7 +209,9 @@ of ftrace. Here is a list of some of the key files:
>  
>  	Set a "trigger" function where tracing should start
>  	with the function graph tracer (See the section
> -	"dynamic ftrace" for more details).
> +	"dynamic ftrace" for more details). As with set_ftrace_filter,
> +	only names of already-loaded functions can be written to
> +	this file.
>  
>    available_filter_functions:
>  

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